Light guide for image sensor

ABSTRACT

A new method to form an image sensor device is achieved. The method comprises forming an image sensing array in a substrate comprising a plurality of light detecting diodes with spaces between the diodes. A first dielectric layer is formed overlying the diodes but not the spaces. The first dielectric layer has a first refractive index. A second dielectric layer is formed overlying the spaces but not the diodes. The second dielectric layer has a second refractive index that is larger than the first refractive index. A new image sensor device is disclosed.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The invention relates to solid state image sensors, and, moreparticularly, to a method to form light guides for an image sensor toeliminate cross talk between pixels.

[0003] (2) Description of the Prior Art

[0004] Light imaging array devices are used in a wide variety ofapplications. These devices utilize an array of active, semiconductordevices, such as diodes, to convert images into streams of digital data.

[0005] Referring now to FIG. 1, a prior art image sensor device isillustrated in cross section. This device comprises a semiconductorsubstrate 10 onto which an array of diodes is formed. Each diodecomprises, for example, an n-type region 22 in a p-type region 18. Eachdiode is separated from other diodes in the array by isolation regions14 to thereby form an array of pixels. The pixels are used to convertincoming light 50 and 54 from a light/image source 46 into electricalsignals via the photo-diodes.

[0006] Typically, the substrate 10 is protected by a series ofdielectric layers 26, 30, and 34. These dielectric layers will transmitthe incoming light 50 and 54 to the pixels 58 and 62. Further, thesedielectric layers may comprises intermetal dielectric (IMD) layers forthe integrated circuit device wherein metal lines 38 and 42 are formed.

[0007] Incident light 50 and 54 will strike the surface of the top mostdielectric layer 34. This light will then be transmitted through theunderlying dielectric layers 30 and 26 down to the underlying pixels. Itis a common occurrence for the incident light 50 and 54 to strike theimaging device surface at a variety of angles. For example, part of thelight 50 strikes the surface at nearly a perpendicular angle. Anotherpart of the light 54 strikes the surface at a clearly non-perpendicularangle. The light 50 that strikes the surface at a near perpendicularangle is transmitted to the pixel 58 underlying the strike location.This is optimal for image sensing performance. However, light 54 thatstrikes the surface at a non-perpendicular angle may then be transmittedto a nearby pixel 62 rather than to the pixel 58 directly underlying thestrike surface. This effect is called cross talk. During a cross-talkevent, light 54 falls on an incorrect pixel diodes 62 rather than theintended pixel diodes 58 due to light scattering. The light scatteringproblem causes degraded image resolution for black and white sensors orcomplicated color correction for color sensors.

[0008] In some prior art sensor arrays, multiple layers of metal lines42 and 38 are used to create metal shields. These metal shields aredesigned to suppress light scattering between adjacent pixels. However,the use of multiple layer, metal lines 38 and 42 requires that the metallines be isolated, and this limitation causes the pixel size toincrease. A light image sensor device with an improved light shield thatdoes not increase the pixel size is a goal of the present invention.

[0009] Several prior art inventions relate to imaging arrays. U.S. Pat.No. 6,001,540 to Huang et al describes a CCD-based imaging array. Thearray forms microlens by a LOCOS process on a polysilicon layer. Lightshield structures, comprising a layer of WSi, are formed over CCDstructures that surround the photodiodes. U.S. Pat. No. 5,648,874 toSawaki et al discloses an optical apparatus. The apparatus uses a matrixcomprising acryl resin. Light shielding films are formed overlying andunderlying the resin. The light shielding films comprise Cr₂O₃ or blackpaint. U.S. Pat. No. 6,195,196 B1 to Kimura et al describes a flat paneldisplay apparatus. U.S. Pat. No. 6,020,944 to Hoshi teaches a LCDapparatus. The apparatus uses a light guide member comprising ananisotropic member with a refractive index anisotropy and anon-anisotropic member with no refractive index anisotropy.

SUMMARY OF THE INVENTION

[0010] A principal object of the present invention is to provide aneffective and very manufacturable method to fabricate an image sensingarray device in the manufacture of an integrated circuit device.

[0011] A further object of the present invention is to provide a methodto form an image sensing array with light guides for each pixel in thearray.

[0012] A yet further object of the present invention is to form lightguides and light shields for pixels in the array using dielectricmaterials having differing indexes of refraction.

[0013] A yet further object of the present invention is to form lightguides and light shields where the total reflection effect is used toprevent cross talk.

[0014] Another further object of the present invention is to provide animage sensor array device having improved performance.

[0015] In accordance with the objects of this invention, a method toform an image sensor device is achieved. The method comprises forming animage sensing array in a substrate comprising a plurality of lightdetecting diodes with spaces between the diodes. A first dielectriclayer is formed overlying the diodes but not the spaces. The firstdielectric layer has a first refractive index. A second dielectric layeris formed overlying the spaces but not the diodes. The second dielectriclayer has a second refractive index that is larger than the firstrefractive index.

[0016] Also in accordance with the objects of this invention, an imagesensor device is achieved. The device comprises an image sensing arrayin a substrate comprising a plurality of light detecting diodes withspaces between the diodes. An array of light guides overlies thesubstrate. The array of light guides comprises a first dielectric layeroverlying the spaces and a second dielectric layer overlying the diodes.The refractive index of the second dielectric layer is larger than therefractive index of the first dielectric layer. Light incident on thesecond dielectric layer overlying any diode is prevented from strikingany other diode by the first dielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the accompanying drawings forming a material part of thisdescription, there is shown:

[0018]FIG. 1 illustrates a prior art image sensing device, in crosssection, showing the problem of light cross talk between adjacent pixelsin the image array.

[0019]FIGS. 2 through 7 illustrate, in cross section, the preferredembodiment of the present invention.

[0020]FIG. 8 illustrates the improved performance of the preferredembodiment of the present invention.

DESCRIPTION OF THE-PREFERRED EMBODIMENTS

[0021] The preferred embodiments of the present invention disclose amethod to form an image sensor array device. The device uses dielectriclayers having differing indexes of refraction to create light guides andlight shields for each pixel in the array. This method prevents lightcross talk. It should be clear to those experienced in the art that thepresent invention can be applied and extended without deviating from thescope of the present invention.

[0022] Referring now to FIG. 2, the preferred embodiment of the presentinvention is illustrated. Several important features of the presentinvention are shown therein and discussed below. The embodiments of thepresent invention are applied to the formation of a unique, image sensorarray comprising a plurality of diodes formed in a semiconductorsubstrate. The teachings may be further applied to any form of imagesensing array.

[0023] In particular, a substrate 100 is provided. The substrate 100preferably comprises a semiconductor material and, more preferably,monocrystalline silicon. The substrate 100 is divided into an array ofactive regions and isolation regions 104. The isolation regions 104 inthe array preferably comprise shallow trench isolation (STI) 104 formedusing techniques well known in the art. However, any isolation techniquemay be used between the diode-pixels. Diode wells 108 may be formed inthe substrate 104. For example, p-well 108 regions may be formed in thesubstrate 100 for each planned diode. These wells 108 may be formedusing diffusion or ion implantation as is well known in the art.

[0024] Referring now to FIG. 3, a plurality of diodes 120 and 108 areformed in the substrate 100. For example, a plurality of n-type regions120 are formed in the substrate 100 such that the n-type regions 120 arecontained in the p-well regions 108. These n-type cathodes 120 arepreferably formed by selectively implanting ions 116 into the substrate100. A masking layer 112 is preferably formed overlying the substrate100. This masking layer 112 may comprise, for example, a photoresistmaterial that is coated overlying the substrate 100. This photoresistmaterial is exposed through actinic light through a mask and developed.After development, the remaining photoresist material forms the maskinglayer 112 shown. The ion implantation 116 is performed using knownmethods to achieve an optimal ion concentration and depth. Following theion implantation step 116, the masking layer 112 is removed bystripping. This implantation 116, plus any anneal or activationtreatment, completes the formation of the array of diodes 108 and 120.Note that an isolation region 104 has been formed between each diode.Therefore, the array actually comprises a plurality of diodes withspaces 104 between the diodes.

[0025] Referring now to FIG. 4, a first dielectric layer 124 and 128 isdeposited overlying the diodes 120 and 108 and the spaces 104 betweenthe diodes. This first dielectric layer 124 and 128 may comprisemultiple levels of material as shown. More preferably, the firstdielectric layer 124 and 128 comprises the intermetal dielectric (IMD)layer used to isolate a plurality of metal levels in the integratedcircuit device. For example, if the device is fabricated using a threemetal level process, then a separate IMD layer would exist for each ofthese metal levels. In the case of a first dielectric layer 124 and 128comprising multiple levels, the various levels 124 and 128 are depositedat different times and may be planarized prior to the deposition of thenext level. In addition, the metal levels would be deposited andpatterned prior to the formation of the dielectric layer for thesubsequent level. These metal levels could be formed using traditionaldeposition and etch or using damascene techniques.

[0026] Of particular importance to the present invention, the firstdielectric layer 124 and 128 must comprise a material having a lowerrefractive index value (n) relative to the refractive index value of asubsequently formed, second dielectric layer, not yet shown. A typicalfirst dielectric layer 124 and 128 material is a doped silicate glass,such as fluorinated silicate glass (FSG). FSG has a low k-value and arelatively low n-value of about 1.3. Further, if the image sensorintegrated circuit device is formed in a 0.18 micron process, then thefirst dielectric layer 124 and 128 should comprise a material that has alow dielectric constant value (k) so that the parasitic capacitance ofthe metal lines is minimized.

[0027] Referring now to FIG. 5, a particularly important feature of thepresent invention is illustrated. The first dielectric layer 124 and 128is patterned. This patterning exposes the underlying diodes 120 byremoving the first dielectric layer 124 and 128 overlying these diodes120. In addition, the first dielectric layer 124 and 128 overlying thespaces 104 between the diodes is not removed. A masking layer 132 may beused to facilitate the selective etching process. This masking layer 132preferably comprises a photoresist material that is patterned using thetechnique described above in FIG. 3. Further the present masking layer132 of FIG. 5 is preferably patterned using the same mask that is usedfor patterning the diode implant masking layer 112 of FIG. 3. After themasking layer 132 is patterned, the first dielectric layer 124 and 128is selectively etched to generate openings 136 that expose the diodeswhile leaving the spaces between 104 covered by the remaining firstdielectric layer 124 and 128.

[0028] Referring now to FIG. 6, a second dielectric layer 140 isdeposited overlying the first dielectric layer 124 and 128, the diodes120, and filling the openings created in the previous step. The seconddielectric layer 140 must comprise a material that will transmit lightto the diodes 120. Most importantly, the second dielectric layer 140must comprise a material having a higher refractive index (n) than thefirst dielectric layer 124 and 128. For example, the second dielectriclayer 140 may comprise a silicon oxide layer deposited using plasmaenhanced CVD and a TEOS source. This exemplary material may exhibit arefractive index of about 1.45. This is substantially higher than therefractive index of the exemplary first dielectric layer 124 and 128comprising FSG that has a value of about 1.3. In general, the refractiveindex of the second dielectric layer 140 should be larger than therefractive index of the first dielectric layer 124 and 128 by at least0.1.

[0029] Referring now to FIG. 7, another important feature of the presentinvention is illustrated. The second dielectric layer 140 is planarizedto the top surface of the first dielectric layer 124 and 128. Thisplanarization step may be performed using any planarizing process knownin the art. However, the planarizing preferably comprises a chemicalmechanical polish (CMP). The planarizing step completes the formation ofthe unique light guides 150 and light shields 154 of the presentinvention by removing the second dielectric layer 140 overlying thefirst dielectric layer 124 and 128.

[0030] The unique method of the present invention creates light guides150 overlying the n-type regions 120 of each of the diodes. These lightguides 150 comprise a material with a relatively large refractive index(n). In addition, light shields 154 are formed overlying the spaces 104between each of the diodes. These light shields comprise material with alower refractive index (n).

[0031] Referring now to FIG. 8, the optical performance of the presentinvention is illustrated. Incident light 164 is emitted from a lightsource 160. This incident light 164 strikes the light guide 150 above apixel. Further, this incident light beam 164 strikes the light guide 150at a non-perpendicular angle and reaches the interface 172 between thelight guide 150 and the light shield 154. At this interface 172, theincident light 164 will make a transition between the high refractiveindex material 140 and the low refractive index material 124 and 128.

[0032] As is well known in the art of optics, the transmission of lightacross such an interface 172 is governed by the equation:

n ₁ sin θ₁ =n ₂ sin θ₂,

[0033] where, in this case, n₁ is the refractive index of the firstdielectric layer 140, n₂ is the refractive index of the seconddielectric layer 124 and 128, θ₂ is the incident angle of the light 164striking the interface 172, and θ₁ is the angle of light transmittedinto the first dielectric layer 124 and 128. Further, if the incidentlight 164 does not strike the interface 172 at a 90 degree angle, then apart of the light will be reflected back into the second dielectriclayer 140. In the present invention case, however, it is not possiblefor the incident light 164 to strike the interface 172 at a 90-degreeangle. Therefore, there must be a reflected light component 168.

[0034] A significant feature of the present invention is the fact thatthe refractive index of the second dielectric layer (n₂) is larger thanthe refractive index of the first dielectric layer (n₁). Therefore,there exists an incident critical angle, θ_(c), where all of theincident light 164 will be reflected back into the light guide material140 and none of the light will be transmitted into the first dielectriclayer 124 and 128. This critical angle, θ_(c), may be found by settingthe transmitted light angle to 90 degrees and results in the equation:

sin θ_(c) =n ₁ /n ₂.

[0035] Based on the refractive indexes of the materials of the preferredembodiment of the present invention, the incident light 164 would betotally reflected back into the second dielectric 140 if the incidentangle θ₂, with respect to the perpendicular of the interface 172surface, exceeds about 61 degrees. As a result, the reflected light 168strikes the pixel diode 120 underlying the incident light guide 150. Thelight shield 154 effectively prevents incident light from reachingadjacent pixels due to cross talk. As a result, the unique constructionof the present invention results in an image sensor device where thelight cross talk effect is dramatically reduced.

[0036] The advantages of the present invention may now be summarized. Aneffective and very manufacturable method to fabricate an image sensingarray device in the manufacture of an integrated circuit device isachieved. An image sensing array is formed having light guides for eachpixel in the array. Light guides and light shields for pixels in thearray are formed using dielectric materials having differing indexes ofrefraction. These light guides and light shields cause total reflectionof incident light within the light guides to thereby prevent cross talk.

[0037] As shown in the preferred embodiments, the novel method anddevice of the present invention provides an effective and manufacturablealternative to the prior art.

[0038] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method to form an image sensor devicecomprising: forming an image sensing array in a substrate comprising aplurality of light detecting diodes with spaces between said diodes;forming a first dielectric layer overlying said spaces but not saiddiodes wherein said first dielectric layer has a first refractive index;forming a second dielectric layer overlying said diodes but not saidspaces wherein said second dielectric layer has a second refractiveindex that is larger than said first refractive index.
 2. The methodaccording to claim 1 wherein said step of forming a first dielectriclayer comprises: depositing said first dielectric layer overlying saiddiodes and said spaces; and patterning said first dielectric layer byremoving said first dielectric layer overlying said diodes whilemaintaining said first dielectric layer overlying said spaces; andwherein said step of forming a second dielectric layer comprises:depositing said second dielectric layer overlying said first dielectriclayer and said diodes; and patterning said second dielectric layer byremoving said second dielectric layer overlying said first dielectriclayer while maintaining said second dielectric layer overlying saiddiodes wherein light incident on said second dielectric layer overlyingany said diode is prevented from striking any other said diode by saidfirst dielectric layer.
 3. The method according to claim 2 wherein saidstep of patterning said first dielectric layer uses the samephotolithographic mask that is used for defining ion implantation areasfor said diodes.
 4. The method according to claim 2 wherein said step ofpatterning said second dielectric layer comprises planarizing saidsecond dielectric layer.
 5. The method according to claim 4 wherein saidplanarizing comprises chemical mechanical polish.
 6. The methodaccording to claim 1 wherein said first dielectric layer comprisesmultiple levels.
 7. The method according to claim 1 wherein said diodescomprise n-type regions in p-type regions.
 8. The method according toclaim 1 wherein said second refractive index is larger than said firstrefractive index by at least about 0.1.
 9. A method to form an imagesensor device comprising: forming an image sensing array in a substratecomprising a plurality of light detecting diodes with spaces betweensaid diodes; depositing a first dielectric layer overlying said diodesand said spaces; patterning said first dielectric layer by removing saidfirst dielectric layer overlying said diodes while maintaining saidfirst dielectric layer overlying said spaces; depositing a seconddielectric layer overlying said first dielectric layer and said diodeswherein said second dielectric layer comprises a refractive index thatis larger than the refractive index of said first dielectric layer; andpatterning said second dielectric layer by removing said seconddielectric layer overlying said first dielectric layer while maintainingsaid second dielectric layer overlying said diodes wherein lightincident on said second dielectric layer overlying any said diode isprevented from striking any other said diode by said first dielectriclayer.
 10. The method according to claim 9 wherein said diodes comprisen-type regions in p-type regions.
 11. The method according to claim 9wherein said second refractive index is larger than said firstrefractive index by at least 0.1.
 12. The method according to claim 9wherein said step of patterning said second dielectric layer comprisesplanarizing said second dielectric layer.
 13. The method according toclaim 12 wherein said planarizing comprises chemical mechanical polish.14. The method according to claim 9 wherein said step of patterning saidfirst dielectric layer uses the same photolithographic mask that is usedfor defining ion implantation areas for said diodes.
 15. The methodaccording to claim 9 wherein said first dielectric layer comprisesmultiple levels.
 16. A method to form an image sensor device comprising:forming an image sensing array in a substrate comprising a plurality oflight detecting diodes with spaces between said diodes; depositing afirst dielectric layer overlying said diodes and said spaces; patterningsaid first dielectric layer by removing said first dielectric layeroverlying said diodes while maintaining said first dielectric layeroverlying said spaces; depositing a second dielectric layer overlyingsaid first dielectric layer and said diodes wherein said seconddielectric layer comprises a refractive index that is higher than therefractive index of said first dielectric layer; and planarizing saidsecond dielectric layer to remove said second dielectric layer overlyingsaid first dielectric layer while maintaining said second dielectriclayer overlying said diodes wherein light incident on said seconddielectric layer overlying any said diode is prevented from striking anyother said diode by said first dielectric layer.
 17. The methodaccording to claim 16 wherein said second refractive index is greaterthan said first refractive index by at least 0.1.
 18. The methodaccording to claim 16 wherein said step of planarizing said seconddielectric layer comprises chemical mechanical polishing.
 19. An imagesensor device comprising: an image sensing array in a substratecomprising a plurality of light detecting diodes with spaces betweensaid diodes; and an array of light guides overlying said substratewherein said array of light guides comprises a first dielectric layeroverlying said spaces and a second dielectric layer overlying saiddiodes, wherein the refractive index of said second dielectric layer islarger than the refractive index of said first dielectric layer, andwherein light incident on said second dielectric layer overlying anysaid diode is prevented from striking any other said diode by said firstdielectric layer.
 20. The device according to claim 19 wherein saiddiodes comprise n-type regions in p-type regions.
 21. The deviceaccording to claim 19 wherein said second refractive index is largerthan said first refractive index by at least 0.1.
 22. The deviceaccording to claim 19 wherein said second dielectric layer is planarizedto the top surface of said first dielectric layer.
 23. The deviceaccording to claim 22 wherein said second dielectric layer is planarizedto the top surface of said first dielectric layer using a chemicalmechanical polish.
 24. The device according to claim 19 wherein saidfirst dielectric layer is patterned using the same photolithographicmask that is used for defining ion implantation areas for said diodes.25. The device according to claim 19 wherein said first dielectric layercomprises multiple levels.